The invention relates to a housing for an electric or electronic component, in particular a motor vehicle control unit, including a one-part or multi-part container and one or more one-part or multi-part sealing devices.
Electric and electronic components, in particular electronic control units, as a rule are mounted inside a housing to protect them against environmental influences, such as mechanical effects, dust, and humidity. If the housing is used in a motor vehicle, these requirements are particularly high. Housings of this type must protect the sensitive electronic components from moisture, which can enter the housing in liquid form as water or in gaseous form as water vapor. As a rule, the housings are made at least in part of a thermoplastic synthetic material. These synthetic parts are watertight, but not gas-tight, so that water in a gaseous form can penetrate. If a multi-part housing is sealed watertight with corresponding seals, no liquid can penetrate, but gas can reach the inside of the housing. The following applies in that case: ##EQU1##
Q: Amount of gas penetrating in cm.sup.3 or amount of water vapor in g PA1 A: Surface in cm.sup.2 PA1 t: Effective time in days PA1 d: Wall thickness in cm PA1 P: Permeation constant in cm.sup.2 /t (gas) or g/(cm*t) (water vapor).sup.1 PA1 Q: Amount of gas penetrating in cm.sup.3 or amount of water vapor in g PA1 A: Surface in cm.sup.2 PA1 t: Effective time in days PA1 .delta.p: Difference in pressure in cm mercury column PA1 d: Wall thickness in cm PA1 P: Permeation constant in cm.sup.2 /t (gas) or g/(cm.sup.2 *t) (water vapor) PA1 P=2.1 10.sup.-6 g/(cm.sup.2 t) for polybutylene terephthalate PA1 A=32 cm.sup.2 PA1 d=1.5 mm PA1 t=15 years=5475 days wherein:
However, if the pressure outside of the housing differs from the pressure on the inside of the housing, which is generated above all by the temperature difference between the inside of the housing and the outside of the housing, air can reach the inside of the housing. The following applies in that case: ##EQU2##
However, the air also contains gaseous water vapor, which penetrates the synthetic material and enters the housing because of its smaller particle size and owing to the pressure difference. Given the following parameters, a non-negligible amount of water can accumulate in the housing in the motor vehicle region. This is true for a relative humidity level of 85% on the outside and 0% relative humidity on the inside and a temperature of 20.degree. C. and 1 bar air pressure.
10% operating time (547.5 days) at 20.degree. C. no pressure difference 20% operating time (1095 days) at 40.degree. C. .delta.p = 0.068 bar = 5.17 cm 50% operating time (2737.5 days) at 60.degree. C. .delta.p = 0.137 bar = 10.41 cm 20% operating time (1095 days) at 80.degree. C. .delta.p = 0.2 bar = 15.2 cm Q.sub.total = Q.sub.20.degree. C. + Q.sub.40.degree. C. ++ Q.sub.60.degree. C. + Q.sub.60.degree. C. = 0.25 g + 2.5 g + 12.8 g + 7.5 g = 23 g
It means that over the total service life of a motor vehicle, in the worst case 23 g water enter through a synthetic housing part with a surface of 32 cm.sup.2 and a thickness of 1.5 mm. The condensed water can do considerable damage to the electronic components. For example, it changes the conductivity between the various components, which can lead to short-circuits and causes oxidation, meaning rusting, of contacts. For those reasons, housings were provided until now with so-called pressure-equalizing openings, where a pressure compensation can occur between the housing inside and the environment. To prevent water from entering at the openings in the housing, these were closed off with a watertight and breathable foil, e.g. as known under the name Goretex or Sympatex, so that the electronic circuit is protected against water damage and moisture damage. This state of the technology is reflected in the German Patent Application No. 197 08 116.9.
It is disadvantageous in this case that an additional operational step is required for closing off the pressure-compensation openings with the watertight and breathable foil. Such watertight and breathable foils are very expensive. Furthermore, it is a disadvantage of such foils that they reduce the diffusion rate as a result of the pressure compensation, to be sure, but are also water vapor permeable.